During the measurement of bioelectric signals, (e.g., of ECG signals), common-mode interference signals (e.g., interference as a result of common-mode signals) occur as a result of non-ideal measurement inputs of an ECG measuring arrangement. These signals arise, for example, from the power supply frequency at 50 Hz. Common-mode interference signals occur if non-identical conditions such as different impedances and capacitances occur at the two measurement inputs during the differential ECG signal measurement. An example of a conventional measuring arrangement for measuring an electrocardiogram is depicted in FIG. 1.
Common-mode signals, (e.g., interference signals), are not concomitantly amplified during the differential measurement, and so they are rejected. The different impedances of the inputs of the ECG measuring arrangement have the effect that different input signals caused by the same interference signal are present at the two inputs of an amplifier circuit of an ECG measuring arrangement, and so the interference signal is amplified together with the actual measurement signal. These common-mode interference signals are very strong in the application on a patient, (for example, a human being or an animal), since the electrode contacts on the patient's skin vary greatly in quality without complex preparation. An electrode contact on the patient may have impedances of between 10 kohms and several megaohms and likewise greatly varying capacitances. As a result, the difference between the impedances and capacitances at two measurement inputs is also in the range of up to several megaohms. An example of an ECG signal subjected to common-mode interference due to an impedance difference of 500 kohms is depicted in FIG. 2. In some instances, the differences in impedance at the inputs of the ECG measuring arrangement are even higher, such that an evaluation of the ECG signal scarcely appears to be possible any longer.
A possible circuit with which the described common-mode interference signals may be determined and rejected is described in German patent application DE 10 2014 219 943 A1. The measuring circuit described in the cited patent application (see FIG. 3) has a first measuring path and a second measuring path. It has in one of the two measuring paths, for example, the second measuring path, a shunt resistor. A voltage drop that is proportional to the common-mode current flowing in the second measuring path occurs at the shunt resistor. In addition, the arrangement includes an adaptive filter, which is set in accordance with the voltage drop detected and filters the detected measurement signal in such a way that the common-mode component of the measurement signal detected is rejected.
The shunt resistor influences the measurement signals detected by way of the amplifier circuit by thermal noise, however.
An alternative arrangement in German patent application DE 10 2014 219 943 A1 includes a shunt resistor in an additional measuring path that is separate from the second measuring path or branches off from it (see FIG. 4). Since in the case of this variant the shunt resistor does not lie directly in the second measuring path, it also does not influence the measurement signals detected by way of the first amplifier circuit by thermal noise.
A problem with the arrangements described is that the adaptive filtering used therein also leads to an attenuation of the useful signal, which reduces the overall gain of the signal/noise ratio. Furthermore, also in the case of the circuit arrangement depicted in FIG. 4, there is still a direct electrical connection between the shunt resistor for measuring the common-mode signals and the second measuring path, even if in this case the shunt resistor is not integrated in the second measuring path directly or in series. The shunt resistor therefore still lies in the range of influence of the analog input circuitry, and so there is still a certain remaining interfering interaction between the measuring arrangement for measuring the common-mode currents and the measuring circuit for measuring the useful signals.